Which Of The Following Is Not An Intrusive Igneous Body

Which of the following is NOT an intrusive igneous body? Understanding Igneous Rock Formations

Igneous rocks, formed from the cooling and solidification of molten rock (magma or lava), are broadly classified into two categories based on their formation: intrusive and extrusive. Understanding the difference is crucial for geologists and anyone interested in Earth's geological processes. This article delves into the characteristics of intrusive igneous bodies, highlighting which formations are not considered intrusive, and explaining the processes that lead to their creation. We will explore the various types of igneous intrusions, contrasting them with extrusive formations, and clarifying common misconceptions.

Defining Intrusive Igneous Bodies: A Deep Dive

Intrusive igneous rocks, also known as plutonic rocks, are formed from magma that cools and solidifies beneath the Earth's surface. This slow cooling process allows for the growth of large crystals, resulting in a phaneritic texture (visible crystals to the naked eye). The pressure and slow cooling rate are key factors in the formation of these bodies, which can vary significantly in size and shape.

Characteristics of Intrusive Igneous Bodies:

• Slow Cooling: The key differentiator is the slow rate of cooling. This slow cooling is a direct result of the insulation provided by the surrounding rock layers. The slower the cooling, the larger the crystals that can form.
• Phaneritic Texture: The large crystal size is a hallmark of intrusive igneous rocks. This texture is readily observable without magnification.
• Variety of Shapes and Sizes: Intrusive bodies range from small dikes and sills to massive batholiths, covering a vast spectrum of scales.
• Depth of Formation: These bodies are formed at significant depths within the Earth's crust, often kilometers below the surface.
• Exposure Through Uplift and Erosion: Intrusive bodies are only exposed at the surface through extensive uplift and erosion processes that remove the overlying rock layers.

Types of Intrusive Igneous Bodies: A Detailed Look

Several types of intrusive igneous bodies exist, each defined by its shape and relationship to the surrounding rock formations.

1. Batholith: The Giant Among Intrusions

Batholiths are the largest intrusive igneous bodies, typically exceeding 100 square kilometers in area. They are massive, irregular-shaped intrusions that represent the roots of ancient volcanic systems. Examples include the Sierra Nevada Batholith in California and the Idaho Batholith.

2. Stock: A Smaller Cousin to the Batholith

Stocks are similar to batholiths but smaller in size, generally less than 100 square kilometers. They also possess irregular shapes and are considered smaller portions of larger magma chambers.

3. Dike: Cutting Through the Layers

Dikes are tabular (flat and sheet-like) intrusions that cut across the bedding planes of the surrounding rock layers. They are typically vertical or steeply inclined and can extend for considerable distances.

4. Sill: Following the Layers

Sills, in contrast to dikes, are concordant intrusions, meaning they are parallel to the bedding planes of the surrounding rocks. They are typically horizontal or gently dipping.

5. Laccolith: Mushroom-Shaped Intrusions

Laccoliths are dome-shaped intrusions that force their way between layers of sedimentary rock, creating a mushroom-like structure. The pressure of the magma causes the overlying layers to bulge upwards.

6. Phacolith: Occupying Folds

Phacoliths are lens-shaped intrusions that occupy the crests of anticlines (upward folds) or the troughs of synclines (downward folds) in layered rock formations.

Extrusive Igneous Bodies: A Contrast to Intrusive Formations

Extrusive igneous rocks, also known as volcanic rocks, form from lava that cools and solidifies on the Earth's surface. The rapid cooling rate prevents the formation of large crystals, resulting in an aphanitic texture (crystals too small to be seen without magnification) or sometimes a glassy texture.

Characteristics of Extrusive Igneous Bodies:

• Rapid Cooling: The rapid cooling is the key differentiator, leading to smaller crystal sizes.
• Aphanitic or Glassy Texture: The texture reflects the rapid cooling.
• Volcanic Features: Extrusive bodies are associated with volcanoes and other volcanic landforms.
• Surface Formation: These rocks are formed directly at the Earth's surface.

Which of the following is NOT an intrusive igneous body? Examples and Explanations

Now, let's address the core question: which of the following is NOT an intrusive igneous body? To answer this, we need a list of potential formations. Let's assume the list includes: Batholith, Dike, Sill, Laccolith, Volcanic Neck, and Extrusive Lava Flow.

The answer is: Volcanic Neck and Extrusive Lava Flow are NOT intrusive igneous bodies.

• Volcanic Neck: A volcanic neck is formed by the solidified magma within the throat of a volcano. While it is igneous, its formation is associated with volcanic activity and the cooling of magma after it has reached the surface. This process clearly defines it as an extrusive feature.

• Extrusive Lava Flow: An extrusive lava flow is the most straightforward example of an extrusive igneous body. Lava cools and solidifies on the Earth's surface, resulting in the rapid formation of aphanitic volcanic rocks. The very definition of an extrusive rock is the opposite of intrusive.

Batholiths, dikes, sills, and laccoliths are all formed deep within the Earth and are prime examples of intrusive igneous bodies.

Common Misconceptions Regarding Igneous Rock Formation

Several misconceptions frequently surround the formation and classification of igneous rocks:

• All Igneous Rocks are Volcanic: This is incorrect. A significant portion of igneous rocks are intrusive, formed beneath the surface.
• Intrusive Rocks Always Contain Large Crystals: While slow cooling typically results in large crystals, factors like the composition of the magma can influence crystal size. Some intrusive bodies may have finer-grained textures than expected.
• Extrusive Rocks Never Contain Crystals: This is also inaccurate. Some extrusive rocks do contain small crystals, though they are generally too small to see without magnification. The presence of phenocrysts (larger crystals within a finer-grained matrix) can be found in some extrusive rocks.

Conclusion: A Comprehensive Understanding

Understanding the difference between intrusive and extrusive igneous bodies is fundamental to comprehending the Earth's geological processes. Intrusive rocks, formed from the slow cooling of magma beneath the surface, display characteristic phaneritic textures and diverse shapes and sizes. Contrastingly, extrusive rocks form from the rapid cooling of lava at the surface, resulting in aphanitic or glassy textures. By recognizing these key differences, we can better appreciate the complex geological history recorded within the rocks around us. Remember, identifying specific intrusive and extrusive formations helps us unravel the intricate story of our planet's dynamic geological past.